In a hitherto employed electric power steering apparatus, an electric motor is driven according to steering torque applied to a handle (a steering wheel) by a driver, thereby imparting steering assistance power to a steering mechanism of a vehicle. Although a brush motor has heretofore, widely been used for the electric motor of the electric power steering apparatus, a brushless motor is also used in recent years in view of enhancement of reliability and durability and a reduction in inertia.
In order to control torque developing in the motor, a motor controller generally detects an electric current flowing through the motor, to thus perform PI control (proportional-plus-integral control) operation in accordance with a difference between an electric current supplied to a motor and a detected electric current. In order to detect an electric current of two phases or more, the motor controller for driving a three-phase brushless motor is provided with two or three current sensors.
In relation to the present invention, Japanese Patent Publication No. 2001-187578A discloses determining a d-axis command voltage and a q-axis command voltage by use of a circuit equations of a motor. Japanese Patent Publication No. 2000-184773A discloses correction of a d-axis command current according to a temperature of a motor.